The invention relates to a method and a device for limiting radiation applied to an object (input dose rate) during irradiation of the object by means of a radiation source, notably during X-ray exposures. The invention also relates to a method and a device for X-ray exposure control while utilizing a method or a device for limiting the input dose rate. The invention also relates to an X-ray generator for carrying out a method of the kind set forth as well as to a corresponding X-ray installation or an X-ray system comprising a device of the kind set forth.
Generally speaking, X-ray installations or systems include one or more X-ray sources, provided with an X-ray tube for generating X-rays, as well as an X-ray generator which is provided with a voltage supply (high-voltage generator) which includes a mains section and delivers the high voltage required for operation of the X-ray tube. When the X-ray source and the components generating the high voltage are combined so as to form one structural unit, such a unit is also referred to as a single-tank generator.
In order to achieve optimum image quality (brightness, contrast, signal-to-noise ratio and sharpness) for the irradiated region, inter alia the intensity of the X-rays incident on the relevant region is of crucial importance. Generally speaking, the image quality is better as the X-ray dose is larger. On the other hand, for obvious reasons it must be ensured that the patient is not exposed to an excessively high dose rate. The maximum value in this respect is imposed by country-specific official rules and regulations.
The dose rate whereto the patient or an object is exposed is dependent essentially on the adjustment of the voltage and the current applied to the X-ray tube, on the distance between the X-ray tube and the object (SSD or Source Skin Distance) as well as on any filters possibly arranged in the beam path between the X-ray tube and the object.
Because these parameters are generally controlled by an automatic exposure device or can be changed by the user during an examination, special steps must be taken so as to ensure that the imposed maximum dose rate whereto the patient is exposed is not exceeded.
EP 1 035 420 discloses a method and a device for exposure control in radiological imaging systems which are intended to solve the problems encountered in respect of the signal-to-noise ratio when the radiation dose on the object is kept constant in the case of a geometrical enlargement of a relevant object due to a change of the distance between an X-ray source and the object or between the X-ray source and an image acquisition device. Therein the input dose is changed in dependence on these two distances in such a manner that a suitable equivalent dose remains constant in the object plane.